The present invention relates to a process for the preparation of a linear tertiary aliphatic carboxyl functional polyester resin, to a linear tertiary aliphatic carboxyl functional polyester resin obtainable by said process, to a polyglycidylester resin obtainable by glycidating said linear tertiary aliphatic carboxyl functional polyester resin and to powder coating compositions comprising said linear tertiary aliphatic carboxyl functional polyester resin or said polyglycidylester resin.
Powder coating compositions based on the solid reaction products of 2,2-bis-(4-hydroxyphenyl)propane and epichlorohydrin have been known for a long time. However, they show only minor ultraviolet resistance and are therefore not suitable for applications requiring a high outdoor durability such as automotive topcoats.
Triglycidylesters which can be used in outdoor durable coatings and in casting compositions are disclosed in U.S. Pat. No. 5,138,078. Due to the secondary nature of the terminal carboxyl functions present in the tricarboxylic acid adduct precursors, strongly alkaline conditions should be avoided during glycidation of these tricarboxylic acid adducts. As a result the triglycidylester produced will contain a relatively high level of hydrolyzable chlorine. In example 2 of U.S. Pat. No. 5,138,078 which relates to the glycidation of the 2:1 adduct of hexahydrophthalic anhydride and dimethylolpropionic acid, the product obtained has a chlorine content of 1.5%. Such a high level of residual chlorine is generally undesirable in coating compositions. In addition, the triglycidylesters reported in U.S. Pat. No. 5,138,078 are liquid, they can not be applied in powder coating compositions.
The most commonly used system for outdoor durable powder coatings are polyester/triglycidylisocyanurate (TGIC). For health and safety reasons however the use of TGIC is not desirable. TGIC is relatively toxic (Rat Oral LD.sub.50 of 0.4 g/kg) and is mutagenic according to the Ames Mutagenicity Test.
Therefore, there is a need for resin systems which are particularly suitable for use in powder coating compositions which are relatively nontoxic as compared to the current polyester/TGIC systems and show excellent outdoor durability in the cured state.
Therefore it is an object of the invention to provide a process for the preparation of a linear acid functional polyester resin containing only tertiary aliphatic carboxylic acid functionality, and/or a polyglycidylester resin thereof suitable for use in outdoor durable powder coating compositions which are relatively environmentally friendly.